I just downloaded and attempted to install the iOS 7 beta, but when I try to activate the device, I'm getting an error:
We’re unable to complete your activation. This device is not registered as part of the iPhone Developer Program. If you are a member of the Program, please register your device in the iPhone Developer Program Portal.
However, I am registered. Here's a screenshot of my devices page: 
Am I missing something?
This solution is working for many people (including myself).
Make sure you actually do add your device [I actually hadn't] (log into apple developers -> members center -> certificates, identifiers, and profiles -> devices -> + button)
Download an old version of iOS (I used this link: http://www.mashinglab.com/download-ios-6-1-4-ipsw-file-for-iphone-5-ipad-ipod-touch-direct-links/)
Put your phone into DFU Mode (http://theiphonewiki.com/wiki/DFU_Mode)
In iTunes, hold down the option button and click restore, and browse for the file you downloaded in step 2.
Wait for it to restore to older version.
In iTunes, hold down the option button and click update (not restore!), and browse for the file you downloaded from the apple developer page for iOS 7.
I think it doesn't work because the developer registration server is under such a heavy load. If you had this problem because you did a restore to iOS 7 then here is a workaround I can confirm works.
This works, because it locally enables the device as a development device so when it upgrades it already knows it is a developer device and doesn't need to check the developer center activation server.
I found a solution to this issue!
And now you just wait and your phone will be activated :)
I found a whole bunch of confirmed solutions in the forum post Unable to complete activation - account not registered.
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